To know if everything's functioning, you need to know an erection works. First, arteries leading to the penis open up and they let the blood rush in and it engorges the penis. "But something else happens that is really subtle," Dr. Oz says. "What drains the penis are these very thin veins. They get squeezed off so the penis can't let the blood come out again. That's why it gets big and hard and stays that way."
This process is controlled by a gas called nitric oxide, which relaxes the arteries and allows the process to start…and is what all those erectile dysfunction drugs treat.